Selective solvent extraction plus filtration of tar sands



Oct. 28, 1969 c. M. GABLE ETAL 3,475,318

SELECTIVE SOLVENT EXTRACTION PLUS FILTRATION 0F TAR SANDS Filed March 28, 1966 INVENTORS! CHARLES M. GABLE EDWARD A. DUNCAN,JR.

E. ROBERT FREITAS THEIR AGENT United States Patent US. Cl. 208-11 7 Claims ABSTRACT OF THE DISCLOSURE Extracting tar low in asphaltenes from a tar sand that contains asphaltenes by treating tar sand with a saturated hydrocarbon solvent having from 5 to 9 carbon atoms per molecule or with a solvent containing saturated hydrocarbons having from 5 to 9 carbon atoms per molecule and up to 20 percent aromatics having 6 to 9 carbon atoms per molecule, and steam stripping the solvent-containing sand after the solvent-tar liquid has been drained therefrom. Steam stripping preferably is accomplished in two stages, the first stage effected by passing steam into a solvent-containing sand bed until just prior to steam breakthrough and the second stage effected by countercurrent contact between a moving bed of sand and a flowing stream of steam.

This invention relates to a method of selectively removing oil from tar sands containing the same. More particularly, this invention relates to a method of selectively removing oil from tar sands by solvent extraction with subsequent solvent recovery.

Large deposits of bituminous containing sand are found in various locations throughout the world and vary considerably in composition and properties. Some are relatively soft and free-flowing while others are very hard and rocklike. The tar content of these sands varies over a wide range and present an attractive source of supply of crude petroleum. For example, one of the largest deposits of tar sands thus far discovered lies in the Athabasca district of Alberta, Canada and extends for many thousands of square miles.

Various methods have been proposed in the past for the recovery of tar from these tar sands but none of the methods proposed has been entirely successful as far as the economy of the operation is concerned. Tar sands suffer the disadvantage of requiring additional processing steps over conventional forms of oil recovery. It is therefore essential that for any process to prove commercially feasable it must be competitive in price with other petroleum sources. It has been proposed to recover the tar content. from tar sands by mixing the sands with water and separating the sand from the aqueous phase of the mixture formed. This process suffers the disadvantage of forming emulsions of water and oil which have been very diflicult to break, thereby resulting in considerable loss of product. Other methods have required slurrying the tar sand in a solvent thereby requiring large quantities of solvent. Still other methods have required the use of a multi-solvent system whereby the tar sand is subjected to a series of solvents before it is finally recovered. In addition, processes heretofore proposed teach the complete removal of tar from the sand. This complete removal may be disadvantageous in that it adds to the processing steps required in refining the thus obtained tar to get the desired oil products. For example, the tar obtained must go through a flash distillation step in which undesirable asphaltenes are removed and the desirable tar products recovered.

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It has now been discovered that the desirable oil products may be selectively removed from the tar sand leaving the undesirable asphaltenes on the sand by solvent extraction with a solvent consisting of a saturated hydrocarbon or mixture of saturated hydrocarbons having from 5 to 9 carbon atoms per molecule.

More particularly, it has been discovered that oil may be selectively removed from tar sands containing the same by extracting them from the tar sand with a saturated solvent consisting of a saturated hydrocarbon or mixture of saturated hydrocarbons having from 5 to 9 carbon atoms per molecule and subsequently recovering the solvent from the product depleted tar sand by steam stripping.

It is sometimes desirable to remove a certain. portion of asphaltenes from the tar sand to use as fuel for the process. The selective removal of some of the asphaltenes along with the oil may be accomplished according to thse present invention by adding to the saturated extraction solvent a controlled amount of a C -C aromatic solvent. In general, saturated hydrocarbon solvents containing about 2 to 20 percent by weight aromatics are sufiicient to extract the desired amount of asphaltenes for fuel. Benzene and toluene are especially preferred as aromatic solvents.

The present invention has the obvious advantages of providing deasphalting and extraction in one step which eliminates or greatly simplifies a subsequent flash distilling step in which undesired asphaltenes are removed, This process also has the advantages of not forming water-tar emulsions as found in the recovery of tar by water addition. Large quantities of solvent are not required as in the slurrying operations and only a single solvent is used for product extraction, thereby essentially eliminating the problems arising with complex solvent systems. For example, solvent recovery by steaming is a relatively simple operation and provides a means for substantially complete solvent removal.

The integrated step of in situ deasphalting and product extraction can best be described by reference to the accompanying drawing which shows a schematic flow diagram of a preferred method of operation.

The tar sand is first stripped of overburden and mined by any appropriate means and brought to an extraction plant for removal of the oil and bituminous materials. This sand generally contains about 5 to 25 percent by weight tar and 1 to 13 percent by weight water. The mined tar sand is fed from source 1 into a crusher 2, which may be a series of crushers or the like, where it is crushed, broken or ground into appropriate sizes for solvent extraction. Generally, particles having a diameter greater than about inch cannot be efficiently extracted. Conversely, particles passing through a #200 mesh screen are disadvantageous in that they tend to plug the filter bed being used during extraction as explained below. Therefore, the tar sand particles are broken down to a size ranging from .003 to .25 inch.

The crushed tar sand is then passed via line 3 which may be a conveyer belt, worm screw, etc., into a solvent extractor 4. This extractor preferably consists of a continuous belt filter, moving pan or rotary pan filter or the like or a series of such filters. The tar sand is uniformly distributed on the filter thereby forming a sand bed of a predetermined thickness, e.g., between 3 to 12 inches. The extraction solvent is then fed onto the bed of tar sand through line 5 and filters through the bed of tar sand extracting the soluble materials from the sand bed. The asphaltenes, being insoluble in the extraction solvent remain in the sand. The fat solvent extract filters through the sand bed and passes out through the bottom of the filter and is recovered in line 6 for further processing. A pressure drop of from about 1 to 10 p.s.i. is maintained across the filter bed by means of applied vacuum in the area immediately under the filter or by means of applied pressures above the bed. In this manner the sand itself acts as a filter and serves to reduce the number of fines or small clay or sand particles that are normally recovered in the oil or bituminous products obtained by other methods. The filtration rates appear to depend somewhat on the particle size distribution of the extracted sand. Tar sand containing a large amount of coarse particless tends to have a faster filtration rate than sand with less of these coarse particles. In addition, sand with high tar concentration filters more slowly, probably because of a viscosity contribution by the tar to the rich solvent. If the sand particles are too coarse, the filtration rates will be too fast to effectively extract the oil and asphalt from the sand. Conversely, if the sand particles are too small or if an excessive amount of fines are present, they tend to plug the filter bed and cause the filtration rate to be excessively slow. Generally, filtration rates of from about 1 to 4 g.p.m./ft. (gallons per minute of solvent per square foot of tar sand on the filter) are preferred. The filtration rate may be varied by adjusting the depth of the sand to be extracted on the filter bed or by reducing or increasing the pressure drop across the bed.

The extraction solvent consists of a saturated hydrocarbon having from 5 to 9 carbon atoms per molecule or mixtures thereof. For example, volatile saturated solvents such as heptane, hexane or a non-aromatic gasoline fraction will selectively remove saturated and aromatic components of the tar from the tar sand while leaving the asphaltenes on the sand. In order to remove the asphaltenes for process fuel it is necessary to add an aromatic solvent such as benzene or toluene to the saturated hydrocarbon. The tar sands usually contain 10 to 35 weight percent asphaltenes based on the weight of the tar on the sand. The amount of aromatic in the solvent needed to extract a certain proportion of the asphaltenes will depend, to a large extent, upon the solvent to original tar ratio used for the extraction. For example, at a volumetric solvent to tar ratio of 3:1 the aromatic concentration of any solvent used will be greatly increased due to the aromatics dissolved out of the oil fraction of the tar. Such a solvent will dissolve a greater portion of asphaltenes than will a saturated solvent having a volumetric solvent to tar ratio of 10:1 since at the higher ratios the dissolved aromatic content of the solvent is not as great and hence the asphaltenes are not as soluble. In general, in order to extract only the oil fraction from the tar sand, a saturated solvent having no initial aromatic solvent is used, and when partial solubility of the asphaltene is desired, solvents containing an aromatic content of from about 2 to percent by weight are sufficient. When using a solvent having a greater degree of aromaticity complete tar removal will be etfected thereby losing the advantages gained by the selective process of the present invention. Complete tar removal by solvent extraction is the basis of copending application Ser. No, 537,903, entitled Separation Process, filed of even date herewith. The amount of solvent added to the tar sand is generally proportional to the amount of tar on the sands. Solvent to original tar ratios between 2:1 and 10:1 by volume may be used with ratios between 3:1 and 6:1 being preferred. The solvent may be added in any manner desired. For example, the extraction process may be carried out in countercurrent stages wherein the tar containing sand is contacted with rich solvent and the fresh lean solvent makes its initial contact with partially oil-depleted sand. In this manner, maximum utilization of the solvent is obtained and consequently lower ratios of solvent to tar are required. An alternative method that may be used is to pre-soak the tar sand in solvent for a short period of time, e.g., 30-60 seconds, before starting the filtration.

The success of any premixing operation may depend to some extent upon the amount of fines in the tar sand. It is postulated that silt or fine particles of sand and clay are encapsulated in the water content of the tar sand with tar surrounding the water as an envelope. If the water capsule is broken during a premixing operation, the fine particles, i.e., those passing through a #200 mesh screen, are released and tend to plug the filter bed during the solvent-tar extraction and hence greatly slow down the filtration rate and increase the amounts of fines in the recovered tar without increasing tar recovery. Therefore filtration of the solvent through the sand bed is preferably carried out without premixing, However, in certain cases wherein the fines content of the sand is relatively low, a short pre-soak or slurrying operation may be an advantageous means of transporting the sand to the filter and dissolving the tar.

For reasons given above, it may also be detrimental to the filtration rate if the water content of the tar sand is removed prior to filtration. A sand containing a high proportion of fines would release these fines onto the bed by the removal of the water in which they are suspended.

The addition of water over and above the normal content of the tar sand may or may not be advantageous to the filtration operation. It is demonstrated in Ser. No. 537,901, entitled Separation Process filed of even date herewith that if just enough water is added to the sand to cause substantially all of the sand grains to be saturated with water, the retention of solvent and tar on the sand will be lessened. However, in some instances, the addition of water causes emulsions to form which are very difiicult to break.

In order to expedite the filtering of the solvent and tar product from the sand filter it is preferred that a pressure drop of about 1 to 10 p.s.i. be maintained across the filter bed of sand. This may be accomplished by maintaining the area under the filter under a suitable vacuum or by means of applied pressure above the bed. The fat solvent extract collected as filtrate contains about to percent of the oil originally present in the tar sand and is passed through line 6 for subsequent solvent separation.

The product-depleted sand containing the asphaltenes and saturated with solvent is then passed via line 7 to a solvent stripping zone 8. The solvent stripper preferably consists of the same type of filter apparatus used in solvent extraction. In a batch operation it can be the same apparatus. The solvent is stripped from the sand by means of steam entering stripping zone 8 through source 9. The steam is pushed or pulled through the sand bed by means of a pressure drop across the filter bed, thereby stripping off the solvent. The degree of solvent recovery is a function of steam quantity and steaming time. Generally, quantities of steam in the range of 3 to 21 pounds of steam per pounds of sand in a period ranging from about 30 seconds to three minutes are sufficient to reduce the solvent content to below 0.01 percent by weight.

The solvent recovered in stripping zone 8 along with water collected from the condensed steam is collected in solent tank 12 by means of lines 10 and 11. The water phase separates from the solvent and is withdrawn through line 29. The recovered solvent is recycled through line 13 back to line 5 where it combines with any makeup solvent required by solvent loss and then enters solvent extractor 4 for additional selective tar extraction. The moist asphaltene-containing sand in stripping zone 8 is substantially depleted of both desirable tar products and solvent and is fed through line 15 to a tailings area or otherwise disposed of.

An alternative method wherein good solvent recovery is attained may be accomplished by passing steam through line 9 into stripping zone 8 until just prior to steam breakthrough at the bottom of the filter bed. The partially steamed sand is then passed via line 14 into rotary kiln drier 16 wherein it is stripped with steam entering line 17 and in a direction essentially countercurrent to'the sand. In this manner, breakthrough of the steam in the sand is delayed, thereby forcing the solvent out ahead of the steam in both units 8 and 16. Once breakthrough of the steam is attained, there tends to be channelling effect within the sand bed thereby lessening the efliciency of the steaming operation. The solvent stripped sand is then passed out of kiln 16 and through line 19 and disposed of as above. The solvent and steam are removed from the kiln through line 11 for separation and eventual recycle to the system.

It is also possible to operate the above-mentioned process in stages. For example, the steam and solvent leaving kiln 16 could be fed by line 11 into line 9 and passed through the sand bed in stripping zone 8, thereby minimizing the amount of steam needed for the overall operation.

The fat solvent extract collected from extractor 4 containing the oil and any asphaltenes extracted is passed through line 6 into solvent fractionator 21. If it is necessary to remove the fines content from the extract prior to solvent fractionation, an extract settling device (not shown) may be inserted between solvent extractor 4 and solvent fractionator 21. Steam enters the fractionator through line 22 and the fractionated solvent is removed through lines 23 and 11 where it joins with the solvent recovered from solvent stripper 8 for water removal and eventual reuse. The bottoms product of fractionator 21 contains the extracted oil and asphaltenes, if any. If no asphaltenes are present, the oil extract may be passed directly to a refinery for processing. If asphaltenes have been extracted to burn for boiler house or process fuel, the residue from fractionator 21 is passed through line 24 into vacuum flasher 25. The tar is vacuum flashed with the aid of steam entering by means of line 26. The flashed distillate, containing the crude oil is then passed via line 27 to a refinery for subsequent processing. The residue in the bottom of flasher 25 contains the asphaltenes and is removed through line 28 and burned as fuel for the process.

The solvent to tar ratio in the extract may vary according to the filterability of the tar sand and the amount of solvent originally required to remove the desired products therefrom. The solvent fractionation is preferably carried out under conventional conditions, e.g., at temperatures of about 350 to 400 F. and a pressure of from about 5 to p.s.i.g. The vacuum flashing operation is carried out under reduced pressure of about 20 to 30 mm. Hg at a temperature of about 750 to 850 F.

While the above description outlines a preferred mode of operating the present process, other modifications are within its scope. A variation of the process described by the drawing may be made by only partially extracting the desired products in a single stage of extractor 4 by a relatively small amount of solvent. Good recovery of both product and solvent can then be obtained by passing steam in a downward direction through the sand as described in stripping zone 8. In this mode of operation, essentially all of the solvent and substantially all of the product to be selectively extracted are displaced ahead of the steam, thereby obtaining a good recovery of tar and solvent at a minimum consumption of steam.

EXAMPLE I A tar sand containing 6.2% weight tar having an asphaltene content of about 31% by weight basis tar was extracted with n-heptane, a non-aromatic solvent. The tar sand was placed on a filter, thereby forming a bed initially 9.5 inches deep. A pressure drop of 7.5 p.s.i. was maintained across the bed by means of applied vacuum. When extracted in two stages with n-heptane at a solventto original tar ratio of 4.2 per stage, a tar recovery of 65% was obtained. A similar two-stage extraction with benzene gave a 97% tar recovery. The difference in recovery indicates that 33% of the tar remained on the sand after the Solvent, percent weight Toluene n-Heptane Solvent/Tar Insolubles From these data it follows that by suitable selection of solvent composition, any desired rejection of asphaltenes may be obtained.

We claim as our invention:

1. A process for extraction and in situ deasphalteneizing of oil from tar sand which comprises contacting tar sand having a particle size of from 0003-025 inch with from 2-10 volumes of solvent per volume of tar of a saturated hydrocarbon solvent having from 5-9 carbon atoms per molecule and limited solubility for asphaltenes, filtering a liquid phase from said sand, steam stripping the solvent-containing sand, separating solvent from the liquid phase and returning it into contact with incoming tar sand, and discarding extracted sand having asphaltenes deposited thereon.

2. The process according to claim 1 wherein said solvent additionally contains up to 20 percent of aromatic hydrocarbons having from 6-9 carbon atoms per molecule wherein asphaltenes are only partially soluble therein.

3. The process according to claim 1 wherein the tar sand is distributed on a filter to form a bed and the oil is selectively extracted from the tar by the solvent filtering through the tar sand bed.

4. The process according to claim 1 wherein extraction is carried out in multiple countercurrent stages.

5. The process according to claim 1 wherein the solvent recovery is carried out in dual stages, the first stage comprising passing the steam into the solvent rich sand bed in a first unit until just prior to steam breakthrough and a second stage comprising passing the sand in a second unit and passing steam through the sand in the second unit in a direction essentially countercurrent to the sand.

6. The process according to claim 1 wherein a pressure drop of from about 1 to 10 p.s.i. is maintained across the sand bed during product extraction and solvent recovery.

7. The process according to claim 1 wherein the extraction solvent is heptane.

References Cited UNITED STATES PATENTS Components of the Bitumen in Athabasca Sand, P. S. Pasternack and K. A. Clark, January 1951, pp. 4-5.

K. A. Clark, volume, Athabasca Oil Sands Information Series No. 45, October 1963, Cottrell, pp. 193205. The Petroleum Chemicals Industry, by Richard Frank Goldstein, second edition, 1958, E. & F. N. Spon., London, pp. 16-19.

DANIEL E. WYMAN, Primary Examiner P. E. KONOPKA, Assistant Examiner 

